Arene-Perfluoroarene-Anion Stacking and Hydrogen Bonding Interactions in Imidazolium Salts for the Crystal Engineering of Polarity

Article abstract of DOI:10.1021/acs.cgd.5b01077

The crystal structure of 1-(2,3,5,6-tetrafluoropyridyl)-3-benzylimidazolium bromide possesses C₆H₅···C₅F₄N···Br^- interactions that link the cations into chains, N(C)C-H···Br^- interactions that link the chains into sheets, and N₂C-H···Br^- interactions that link the sheets to one another. As a consequence of these, it is polar (Pna2₁). Density functional theory calculations indicate that the strength of the interaction between a cation and a bromide anion lies in the order N₂C-H···Br^- > N(C)C-H···Br^- > C₆H₅···C₅F₄N···Br^-. Prevention of the N₂C-H···Br^- interaction by substitution of the hydrogen atom with a methyl group leads to dimers linked by two C₆H₅···C₅F₄N···Br^- interactions. Prevention of the N(C)C-H···Br^- interaction by substitution of the hydrogen with a methyl group permits chains of cations, but because the N₂C-H···Br^- interactions link the chains there are no strong interactions between the sheets. Chains of cations linked by Ar···C₅F₄N···Br^- interactions also arise when the benzyl group is replaced by 3-phenylbenzyl and 2-naphthylmethyl groups. The former also contains N₂C-H···Br^- and N(C)C-H···Br^- interactions and is centrosymmetric. The latter does not contain N(C)C-H···Br^- interactions and is chiral and polar (P2₁). Exchanging the positions of the aryl and polyfluoroaryl groups results in a crystal structure with no π-π stacking between the aryl and polyfluoroaryl groups although N₂C-H···Br^- and N(C)C-H···Br^- interactions persist.

Full text of DOI:10.1021/acs.cgd.5b01077

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Article
Arene-Perfluoroarene-Anion Stacking and Hydrogen Bonding
Interactions in Imidazolium Salts for the Crystal Engineering of Polarity
Hanan I. Althagbi, Alison J Edwards, Brian K. Nicholson, Daniel A. Reason,
Graham C. Saunders, Sophie A. Sim, and Danielle A. van der Heijden
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b01077 • Publication Date (Web): 30 Nov 2015
Downloaded from http://pubs.acs.org on December 11, 2015
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AreneꢀPerfluoroareneꢀAnion Stacking and Hydrogen Bonding Interactions in
Imidazolium Salts for the Crystal Engineering of Polarity
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†
‡
†
†
Hanan I. Althagbi, Alison J. Edwards, Brian K. Nicholson, Daniel A. Reason, Graham C.
†,
†
†
Saunders, * Sophie A. Sim, and Danielle A. van der Heijden
†
School of Science, University of Waikato, Hamilton, 3240, New Zealand
‡
Bragg Institute, Australian Nuclear Science and Technology Organisation, New Illawarra
Road, Lucas Heights, NSW 2234, Australia
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ABSTRACT
The crystal structure of 1ꢀ(2,3,5,6ꢀtetrafluoropyridyl)ꢀ3ꢀbenzylimidazolium bromide
ꢀ
ꢀ
possesses C
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H
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···C
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N···Br interactions that link the cations into chains, N(C)C―H···Br
ꢀ
interactions that link the chains into sheets, and N C―H···Br interactions that link the sheets
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to one another. As a consequence of these it is polar (Pna2
1
). DFT calculations indicate that
the strength of the interaction between a cation and a bromide anion lies in the order
ꢀ
ꢀ
ꢀ
ꢀ
N
2
C―H···Br > N(C)C―H···Br > C
6 5 5 4 2
H ···C F N···Br . Prevention of the N C―H···Br
interaction by substitution of the hydrogen atom with a methyl group leads to dimers linked
ꢀ
ꢀ
by two C H ···C F N···Br interactions. Prevention of the N(C)C―H···Br interaction by
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5 4
substitution of the hydrogen with a methyl group permits chains of cations, but because the
ꢀ
N
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C―H···Br interactions link the chains there are no strong interactions between the sheets.
ꢀ
Chains of cations linked by Ar···C
5
F
4
N···Br interactions also arise when the benzyl group is
replaced by 3ꢀphenylbenzyl and 2ꢀnaphthylmethyl groups. The former also contains
ꢀ
ꢀ
N C―H···Br and N(C)C―H···Br interactions and is centrosymmetric. The latter does not
2
ꢀ
contain N(C)C―H···Br interactions and is chiral and polar (P2
1
). Exchanging the positions
of the aryl and polyfluoroaryl groups results in a crystal structure with no π–π stacking
ꢀ
ꢀ
between the aryl and polyfluoroaryl groups although N
2
C―H···Br and N(C)C―H···Br
interactions persist.
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INTRODUCTION
The offset parallel stacking of polyfluoroarenes with arenes, termed ‘π–π stacking’,
1
has been identified as a useful interaction for crystal engineering. The attraction between
arenes and polyfluoroarenes arises in part from interactions between the ring quadrupoles
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,3
which are of opposite sign. For example, the quadrupole moments of benzene and
ꢀ
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hexafluorobenzene are ꢀ33.3 ± 2.1 × 10 and 31.7 ± 1.7 × 10 C m respectively, and
ꢀ1 5
their interaction, calculated to be ꢀ5.38 kcal mol , results in π–π stacking to give the
archetypal structure (Cambridge Structural Database entry code: BICVUE01): alternating
parallel benzene and hexafluorobenzene molecules arranged in columns inclined at ca.
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ꢀ
6
3° to the planes of the rings, with a separation of ca. 3.35 Å and an offset of ca. 1.7 Å.
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The structures of coꢀcrystals of other arenes and fluoroarenes show similar features.
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π–π Stacking also occurs between complementary compounds with two or more rings,
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21ꢀ27
and compounds containing both an aryl and a polyfluoroaryl group.
In most of the
structures of the latter, the rings of each molecule are coꢀplanar, but this is not necessary
for π–π stacking. In the crystal structures of 2,3,4,5,6ꢀpentafluorobiphenyl (CSD entry
2
6
code: PFBIPH) and both polymorphs of 2ꢀ(pentafluorophenylimino)methylphenol (CSD
2
7
entry codes: BANGOM, BANGOM02), the plane of the polyfluoroaryl ring is twisted
by ca. 53º and ca. 40° respectivley relative to the plane of the aryl ring of the same
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7ꢀ29
molecule. π–π Stacking is also tolerant of some functional groups,
and this has
allowed the use of these interactions to control the stereochemistry of the [2+2]
3
0
photodimerization of stilbenes in the solid state.
If compounds bearing both aryl and polyfluoroaryl rings undergo ‘bifurcated
stacking’, in which each molecule stacks with four others (Figure 1a), sheets with
polarity in one dimension result. If the sheets pack in a parallel manner then this provides
the basis for the generation of crystal polarity, which is a requirement for pyroelectric and
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ferroelectric properties, and also gives rise to piezoelectric and secondꢀorder nonꢀlinear
3
1
optical properties. For molecules in which the complementary rings can be coꢀplanar,
structures containing ‘columnar stacking’, in which each molecule stacks with only two
others
(Figure
1b),
dominate.
Exceptions
include
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2ꢀ(4ꢀnitrilotetrafluorophenyl)benzo[d][1,2,3]triazole (CSD entry code: OKAJAM), which
adopts bifurcated stacking, possibly because of its multicomponent nature
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(
NCꢀC
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F
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ꢀN
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ꢀC
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H
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and
those
components’
dimensions,
and
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methoxytetrafluorophenylꢀ2ꢀphenylacetylene (CSD entry code: ASIJOB) which adopts a
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2
structure with bifurcated stacking to accommodate the methoxy groups. The alternating
direction of the polarity of the molecules arising from columnar stacking does not give
rise to net polarity. Molecules bearing both aryl and polyfluoroaryl rings tend, in
consequence, not to crystallize in one of the polar crystal classes (1, 2, m, mm2, 4, 4mm,
3
, 3m, 6 and 6mm); however, other factors may lead to crystallization in polar space
groups. 2ꢀ(Pentafluorophenylimino)methylphenol crystallizes with columnar stacking in
both the centrosymmetric space group P2 /c (the α2 polymorph, CSD entry code:
1
BANGOM) and the polar space group P2 (the α1 polymorph, CSD entry code:
1
2
7
BANGOM02).
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Figure 1.
(
a
) Bifurcated and (
b
) columnar stacking of molecules containing complementary
polyfluoroaryl and aryl rings. The arrow indicates the direction of polarity.
The design and construction of a polar network incorporating π–π stacking
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interactions between benzyl and pentafluorobenzyl substituents of an imidazolium cation
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3
was recently reported (CSD entry code: BONKUL). It was argued that preventing
columnar stacking, and thereby facilitating the bifurcated stacking of the rings, would
generate nonꢀcentrosymmetry. It was reasoned that columnar stacking could be prevented
by the use of imidazolium cations in which the planes of the complementary rings could
be parallel but not coꢀplanar, and in which the ‘step’ is more than 4 Å. The cation of 1ꢀ
(
2,3,4,5,6ꢀpentafluorobenzyl)ꢀ3ꢀbenzylꢀimidazolium bromide satisfies the criteria, and the
3
3
salt was indeed found to crystallize in the polar (and chiral) space group P1. The cations
undergo bifurcated π–π stacking to produce sheets with polarity in two directions: parallel
to the b and c axes. It is the packing of the sheets that determines the nonꢀ
centrosymmetry, and this is determined by interactions between the bromide anions and
2
cations of adjacent sheets: the N CH hydrogen atom and one other hydrogen atom of each
imidazolium ring interact with bromide anions to orientate the rings, and sheets, parallel,
generating polarity parallel to the a axis.
We hypothesize that prevention of columnar stacking can also be achieved by using
check mark shaped (tick shaped) molecules or ions. Angular, chevron shaped molecules
bearing both polyfluoroaryl and aryl groups have been reported to undergo columnar
2
8,29
stacking.
For 1,2,3,4ꢀtetrafluorodibenzo[b,f][1,4]ꢀoxazepine (CSD entry code:
2
8
HUTYAW) the planes of the phenylene and tetrafluorophenylene rings subtend an angle
o
of ca. 144 at the hinge, and together with the equidistance of the rings from the fold, this
allows the sufficiently close approach of the molecules to give rise to π–π stacking
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interactions and resultant columnar stacking (Figure 2a). If the complementary rings are
not equidistant from the hinge then columnar stacking does not maximize the π–π
stacking interactions (Figure 2b), but bifurcated stacking may do so (Figure 2c). The
validity of this hypothesis was demonstrated recently for 1ꢀ(4ꢀnitriloꢀ2,3,5,6ꢀ
tetrafluorophenyl)benzimidazole which crystallized in the polar space group Cc (CSD
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entry code: PEYDEE).
Since this compound is neutral and contains no groups capable
of hydrogen bonding, the π–π stacking interactions determine the crystal structure.
Figure 2. Columnar stacking of (a) chevron shaped and (b) check mark shaped
molecules, and (c) bifurcated stacking of check mark shaped molecules.
We wished to test this hypothesis for salts; i.e. can crystal polarity be engineered by
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π–π stacking between check mark shaped ions? The polar crystal structure of 1ꢀ
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(
2,3,4,5,6ꢀpentafluorobenzyl)ꢀ3ꢀbenzylꢀimidazolium bromide prompted us to start an
investigation of 1ꢀpolyfluoroarylꢀ3ꢀbenzylimidazolium salts. In addition to the desired π–
π stacking interactions, other interactions are expected. Short distances between carbon
atoms of the imidazolium ring and anions lying in, or close to, the plane of the
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3
3,36ꢀ43
imidazolium ring have been observed for other salts.
The distances and geometries
4
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are indicative of hydrogen bonding,
the importance of which in determining crystal
Calculations performed on 1ꢀethylꢀ3ꢀ
4
7ꢀ49
structures is well established.
methylimidazolium salts suggest that the interactions are not conventional hydrogen
bonds, and are dominated by an isotropic charge–charge term that is nearly inversely
proportional to the distance of the anion from the center of the positive charge, which is
4
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close to the midpoint between the two nitrogen atoms. These interactions were
ꢀ
1
calculated to be attractive by 68 to 82 kcal mol . Crystal structures of the 1ꢀ
50
polyfluoroarylꢀ3ꢀ benzylimidazolium salts may also contain anion–π interactions, which
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1ꢀ56
have been observed in a number of crystal structures of ammonium,
5
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phosphonium,
imidazolium
and other
salts in which the cation bears a
polyfluoroaryl group. Chloride, bromide and iodide typically lie ca. 3.2, 3.4 and 3.7 Å
from the plane of the polyfluroaryl ring and are displaced from the normal to its centroid
towards an atom bearing the positive charge. Theoretical studies have shown that the
interactions between hexafluorobenzene and halide anions are attractive by 10.6 to 20.9
ꢀ
1 50,61ꢀ64
kcal mol ,
and that the energy decreases by only a small extent as the anion is
diplaced from the normal to the centroid of the ring towards an edge. For chloride at 3.05
Å from the plane of the hexafluorobenzene ring energies of interaction were calculated by
the RIꢀMP2(full) method using the augꢀccꢀpVDZ basis set to be ꢀ14.05, ꢀ13.12 and ꢀ13.43
ꢀ
1
kcal mol when it lies on the normals to the centroid, a carbon atom and the midpoint of a
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C–C bond respectively. The interactions are expected to be stronger for polyfluoroaryl
groups that are part of a cation.
Here we report the results of a structural study of six imidazolium bromide salts
with check mark shaped cations bearing both aryl and fluoroaryl substituents. The crystal
structure of one, 1ꢀ(2,3,5,6ꢀtetrafluoropyridyl)ꢀ3ꢀbenzylimidazolium bromide (1), has
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been communicated.
EXPERIMENTAL SECTION
Pentafluoropyridine, 1,2,3,4,5ꢀpentafluorobenzyl bromide (Apollo Scientific), benzyl
bromide,
3ꢀphenylbenzyl
bromide,
2ꢀbromomethylnaphthalene,
imidazole,
2ꢀ
methylimidazole, 4ꢀmethylimidazole and 1ꢀphenylimidazole (Aldrich) were used as supplied.
1
NMR spectra were recorded on Bruker DRX300 or DPX400 spectrometers. H NMR spectra
(300.13 or 400.14 MHz) were referenced internally using the residual protio solvent
1
3
resonance relative to tetramethylsilane (δ 0), C NMR spectra (100.61 MHz) were
1
9
referenced externally to tetramethylsilane (δ 0), and F NMR spectra (282.40 or 376.47
MHz) externally to trichlorofluoromethane (δ 0). All chemical shifts are quoted in δ (ppm)
using the high frequency positive convention, and coupling constants in Hz. Elemental
analyses were performed by the Campbell Microanalytical Laboratory, The University of
Otago. The electron impact mass spectrum of 1ꢀ(2,3,5,6ꢀtetrafluoropyridyl)imidazole was
recorded on a VG Autospec X series and the electrospray ionization mass spectra on a Bruker
Daltonics MicrOTOF spectrometer.
Synthesis of 1ꢀ(2,3,5,6ꢀTetrafluoropyridyl)imidazole. A solution of imidazole (0.30
3
g, 4.4 mmol) and pentafluoropyridine (0.74 g, 4.4 mmol) in THF (50 cm ) was left at ambient
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temperature for 20 hours, during which time a small number of colourless crystals were
deposited. The solvent was removed by rotary evaporation affording a pale yellow oil and
colourless solid, which is presumed to be salts formed by the reaction of hydrogen fluoride,
the byꢀproduct of the reaction, with borosilicate glass. The product was extracted into
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dichloromethane (2 × 50 cm ), and the solution filtered. Concentration by rotary evaporation
1
afforded the product as a pale yellow oil. Yield 2.38 g (62%). H NMR (CDCl
3
): δ = 7.99
13
1
(1H, s, N
2
CH), 7.41 (1H, m, HC=CH),7.32 (1H, m, HC=CH). C{ H} NMR (CDCl ): δ =
3
1
1
1
44.7 (dddd, J = 246 Hz, JCF = 15, 15, 4 Hz, CF), 137.5 (t, JCF = 6 Hz), 135.6 (dm, J
=
CF
CF
19
2
5 4 3
64 Hz, CF), 131.2 (s), 128.1 (m), 119.4 {t, JCF = 4 Hz, NC(C F N)}. F NMR (CDCl ): δ =
ꢀ
86.81 (2F), ꢀ149.07 (2F) A and B parts of an AA'BB' spin system. EI MS: C
8 3 3 4
H N F
requires 217.0263; found 217.0242.
Synthesis of 1ꢀ(2,3,5,6ꢀTetrafluoropyridyl)ꢀ3ꢀbenzylimidazolium bromide (1).
Benzyl bromide (0.56 g, 2.6 mmol) was added to 1ꢀ(2,3,5,6ꢀtetrafluoropyridyl)imidazole
3
(
0.45 g, 2.6 mmol) in dichloromethane (50 cm ) and the solution left at ambient temperature
for 24 h. The solvent was removed by rotary evaporation to afford the product as a white
powder, which was recrystallized from methanol. Yield 0.88 g (87%). Found: C 46.55, H
1
2.61, N 10.81. Calc. for C15
H
10BrN
3
F
4
: C, 46.41, H 2.60, N 10.83%. H NMR {(CD
3
)
2
SO}: δ
=
10.09 (1H, s, N CH), 8.27 (2H, m, HC=CH), 7.55 (2H, m, C H ), 7.46 (3H, m, C H ), 5.67
2 6 5 6 5
1
3
1
1
(
2H, s, CH
2
3 2
). C{ H} {(CD ) SO}: δ = 143.3 (dddd, JCF = 242, JCF = 15, 15, 4 Hz, CF),
1
139.1 (s), 137.3 (dm, JCF = 266 Hz, CF), 134.4 (s), 129.6 (m), 129.2 (s), 126.3 (s), 124.4 (s),
19
121.0 (m), 53.4 (s, CH
2
). F NMR {(CD
3
)
2
SO}: δ = ꢀ88.74 (2F), ꢀ146.69 (2F) A and B parts
+
of an AA'BB' spin system. ESI MS: C H F N requires 308.081; found: [M ꢀ Br] 308.084.
15
10
4
3
Synthesis of 1ꢀ(2,3,5,6ꢀTetrafluoropyridyl)ꢀ2ꢀmethylimidazole. Pentafluoropyridine
(0.85 g, 5.0 mmol) and 2ꢀmethylimidazole (0.39 g, 4.7 mmol) were treated as for the
preparation of 1ꢀ(2,3,5,6ꢀtetrafluoropyridyl)imidazole. The product was obtained as a viscous
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1
yellow oil. Yield 0.99 g (91%). H NMR (CDCl ): δ = 7.19 (1H, s, HC=CH), 7.03 (1H, s,
3
19
HC=CH), 2.40 (3H, m, CH
3
). F NMR (CDCl
3
): δ = ꢀ86.10 (2F), ꢀ146.09 (2F) A and B parts
+
of an AA'BB' spin system. ESI MS: C H N F requires 232.050; found [M + H] 232.050.
8
3
3 4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Synthesis
of
1ꢀ(2,3,5,6ꢀTetrafluoropyridyl)ꢀ2ꢀmethylꢀ3ꢀbenzylimidazolium
bromide (2). 1ꢀ(2,3,5,6ꢀtetrafluoropyridyl)ꢀ2ꢀmethylimidazole (0.42 g, 1.8 mmol) and benzyl
bromide (0.28 g, 1.6 mmol) were treated as for 1. The product was obtained as a white
3 4
powder. Yield 0.49 g (76%). Found: C 47.86, H 2.89, N 10.51. Calc. for C16H12BrN F : C,
1
47.78, H 3.01, N 10.45%. H NMR {CDCl
3
/(CD
3
)
2
SO}: δ = 8.06 (2H, s, HC=CH), 7.47 (5H,
13
1
m, C
6
H
5
), 5.55 (2H, s, CH
2
), 2.72 (s, CH
3
). C {(CD
3
)
2
SO}: δ = 147.9 (s), 143.4 (dm, JCF
=
1
1
2
2
43, CF), 138.4 (dm, JCF = 267 Hz, CF), 133.7 (s), 129.7 (dd, JCH = 161, JCH = 5 Hz), 129.5
1
1
1
(
dm, JCH = 159 Hz), 128.9 (dm, JCH = 160 Hz), 124.9 {m, NC(C
F
5 4
N)}, 124.7 (dd, JCH
=
=
2
1
2
1
207, J = 10 Hz, HC=CH), 123.2 (dd, J = 211, J = 12 Hz, HC=CH), 52.3 (t, J
CH
CH
CH
CH
1
19
146 Hz, CH
2
), 11.0 (quart, JCH = 135 Hz, CH
2
). F NMR {CDCl
3
/(CD
)
3 2
SO}: δ = ꢀ88.14
(
2F), ꢀ144.19 (2F) A and B parts of an AA'BB' spin system. ESI MS: C16H F N requires
12 4 3
+
322.097; found: [M ꢀ Br] 322.095.
Synthesis of 1ꢀ(2,3,5,6ꢀTetrafluoropyridyl)ꢀ4ꢀmethylimidazole. Pentafluoropyridine
(
0.576 g, 3.4 mmol) and 4ꢀmethylimidazole (0.307 g, 3.7 mmol) were treated as for the
preparation of 1ꢀ(2,3,5,6ꢀtetrafluoropyridyl)imidazole. The product was obtained as a viscous
1
pale yellow oil. Yield 0.78 g (100%). H NMR (CDCl ): δ = 7.15 (1H, s, HC=CH), 7.03 (1H,
3
19
m, HC=CH), 2.37 (3H, m, CH
3
). F NMR (CDCl
3
): δ = ꢀ87.06 (2F), ꢀ149.30 (2F) A and B
+
parts of an AA'BB' spin system. ESI MS: C H N F requires 232.050; found [M + H]
9
6
3 4
232.049.
Synthesis
of
1ꢀ(2,3,5,6ꢀTetrafluoropyridyl)ꢀ3ꢀbenzylꢀ4ꢀmethylimidazolium
bromide (3). 1ꢀ(2,3,5,6ꢀtetrafluoropyridyl)ꢀ4ꢀmethylimidazole (0.78 g, 3.4 mmol) and benzyl
bromide (0.64 g, 3.7 mmol) were treated as for 1. The product was obtained as a white
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3
4
5
6
7
8
9
3 4
powder. Yield 0.66 g (49%). Found: C 48.14, H 3.14, N 10.56. Calc. for C16H12BrN F : C,
1
4
7.78, H 3.01, N 10.45%. H NMR {CDCl /(CD ) SO}: δ = 9.99 (1H, s, N CH), 8.08 (1H,
3
3 2
2
1
3
m, HC=C), 7.45 (5H, m, C
6
H
5
), 5.68 (2H, s, CH
2
3 3 2
), 3.35 (s, CH ). C {(CD ) SO}: δ =143.4
1
1
1
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(dm, J = 241, CF), 139.1 (dm, J = 229 Hz), 137.3 (dm, J = 265 Hz, CF), 133.5 (m),
CF
CH
CF
1
1
1
1
33.1 (m), 129.7 (dd, JCH = 162 Hz), 129.4 (dm, JCH = 162 Hz), 128.7 (dm, JCH = 159 Hz),
1
1
126.4 {m, NC(C
5
F
4
2
N)}, 120.9 (dm, JCH = 210 Hz, HC=C), 51.0 (t, JCH = 145 Hz, CH ), 9.9
1
19
(
quart, JCH = 130 Hz, CH
2
). F NMR {CDCl
3
/(CD
3
)
2
SO}: δ = ꢀ88.79 (2F), ꢀ146.87 (2F) A
and B parts of an AA'BB' spin system. ESI MS: C H F N requires 322.097; found:
1
6
12
4
3
+
[
M ꢀ Br] 322.105.
Synthesis
of
1ꢀ(2,3,5,6ꢀTetrafluoropyridyl)ꢀ3ꢀ(3ꢀphenylbenzyl)imidazolium
bromide (4). 3ꢀPhenylbenzyl bromide (0.558 g, 2.26 mmol) was added to 1ꢀ(2,3,4,5ꢀ
3
tetrafluoropyridyl)imidazole (0.488 g, 2.25 mmol) in dichloromethane (30 cm ) and the
solution left at ambient temperature for 24 h. The solvent was removed by rotary evaporation
to give a white powder. Recrystallization from methanol afforded the product as the
monohydrate. Yield 0.394 g (36.5%). Found: C, 52.46; H, 3.25; N, 8.75. Calc. for
1
C
21
H
12BrF
4
N
.H
3 2
O: C, 52.52; H, 2.94; N, 8.75%. H NMR {(CD
3
)
2
SO}: δ = 10.15 (1H, br s,
2
N CH), 8.36 (1H, m), 8.29 (1H, m), 7.93 (1H, m), 7.74 (1H, m), 7.71 (2H, dm, J = 8.2 Hz),
1
9
7
.57 (2H, m), 7.50 (2H, tm, J = 7.7 Hz), 7.41 (1H, m), 5.75 (2H, s, CH2).
F
NMR{(CD
3
)
2
SO}: δ = ꢀ88.86 (2F), ꢀ146.62 (2F) A and B parts of an AA'BB' spin system.
+
ESI MS: C21
H
14
F
4
N
3
requires 384.112; found: [M ꢀ Br] 384.118.
Synthesis of 1ꢀ(2,3,5,6ꢀTetrafluoropyridyl)ꢀ3ꢀ(2ꢀnaphthylmethyl)imidazolium
bromide (5). 2ꢀBromomethylnaphthalene (0.424 g, 1.92 mmol) was added to 1ꢀ(2,3,5,6ꢀ
3
tetrafluoropyridyl)imidazole (0.415 g, 1.91 mmol) in dichloromethane (50 cm ) and the
solution left at ambient temperature for 24 h. The solvent was removed by rotary evaporation
to give a yellow oil. Recrystallization from methanol afforded yellow crystals of the product
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1
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2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
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4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
as the monohydrate. Yield 0.762g (87.4%). Found: C 49.35, H 3.01, N 9.13. Calc. for
1
C
19
H
12BrF
4
N
.H
3 2
O: C, 50.02, H 3.09, N 9.21%. H NMR (CD
3
2
Cl): δ = 11.69 (1H, s, N CH),
19
8.13 (1H, s, HC=CH), 7.90 (3H, m), 7.65 (3H, m), 7.59 (2H, m), 6.13 (2H, s, CH
2
). F NMR
(
CD Cl): δ = ꢀ83.05 (2F, m), ꢀ145.97 (2F, m) ppm A and B parts of an AA'BB' spin system.
3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
+
ESI MS: C H F N requires 358.097; found: [M ꢀ Br] 358.104.
1
9
12
4
3
66
Synthesis of 1ꢀ(2,3,4,5,6ꢀPentafluorobenzyl)ꢀ3ꢀphenylimidazolium bromide (6).
2,3,4,5,6ꢀPentafluorobenzyl bromide (0.67 g, 2.57 mmol) was added to 1ꢀphenylimidazole
3
(
0.31 g, 2.15 mol) in dichloromethane (20 cm ) and the solution left at ambient temperature
for 24 h. The solvent was removed by rotary evaporation to afford the product as a white
1
powder. Yield 0.88 g (ca. 100%). H {(CD ) SO}: δ = 9.91 (1H, s, N CH), 8.40 (1H, t, J =
3
2
2
6 5 6 5
1.9 Hz, HC=CH), 8.10 (1H, t, J = 1.9 Hz, HC=CH), 7.81 (2H, m, C H ), 7.64 (3H, m, C H ),
19
3
5.72 (2H, s, CH
2
). F {(CD
3
)
2
SO}: δ = ꢀ140.38 (2F, m, Fortho), ꢀ153.05 (1F, t, J = 22.3 Hz,
+
10 5 2
Fpara), ꢀ161.84 (2F, m, Fmeta). ESI MS: C16H F N requires 325.076; found: [M ꢀ Br]
325.074.
Singleꢀcrystal Xꢀray Diffraction Structure Determination. Crystals of 1, 3, 4 and
were grown from methanol, and crystals of 2 and 6 were grown from dichloromethane.
5
Unit cell dimensions and reflection data for 1 and 3 ꢀ 6 were recorded on a Bruker Nonius
Apex II CCD diffractometer at 89(2) or 90(2) K using Mo Ka radiation. Absorption
6
7
corrections to the data were made by SADABS. Crystal and refinement data are presented
6
8
in Table 1. The structures were solved by direct methods using SHELXSꢀ97. The structures
69
70
of 1, 3, 4 and 6 were refined using SHELXLꢀ97 and that of 5 using CRYSTALS with
nonꢀhydrogen atoms anisotropic. All hydrogen atoms were included in calculated positions.
1
For 5 initial refinement reached only R = 0.14 with several large, obviously spurious, peaks
in difference maps. Since the angle β was close to 90°, twinning by pseudoꢀmerohedry was
71
suspected and treatment of the data using the ROTAX procedure incorporated in
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2
CRYSTALS revealed the twin operator {100/0ꢀ10/ꢀ0.0080ꢀ1}. Data were reꢀindexed to
assign reflections either to the sum of the components or to the dominant component and
refinement of the model with suitable distance restraints was successful converging at a 76:24
twin ratio. Hydrogen atoms were included in the refinement for water of solvation. The final
10
11
12
13
14
15
16
17
18
19
20
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22
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24
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43
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45
46
47
48
49
50
51
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53
54
55
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59
60
model gave R of 0.040, with no significant residual electron density.
1
Unit cell dimensions and reflection data for 2 were recorded on an Agilent
SuperNova, single source at offset, Atlas diffractometer at 101.3(5) K using Cu K
α
radiation.
7
3
74
Using Olex2 the structure was solved with the olex2.solve structure solution program
74
using Charge Flipping and refined with the olex2.refine refinement package using Gaussꢀ
7
5
Newton minimization. Structure and packing diagrams were generated using ORTEP3v2
76
and Mercury respectively and are shown with displacement ellipsoids at the 50% level.
CIF files have been deposited with the Cambridge Structural Database (CCDC
reference numbers 806231 (1), 1024878 (2), 976401 (3) and 934194–934196 (4 ꢀ 6)) and can
be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
(
+44) 1223−336−033; or deposit@ccdc.cam.ac.uk).
Singleꢀcrystal Neutron Diffraction Structure Determination. Laue neutron
7
7
diffraction data were collected for 1 using the KOALA instrument on a thermal neutron
guide at the OPAL reactor source, ANSTO, Australia. After screening, a suitable single
crystal (0.30 × 0.60 × 0.67 mm) was mounted on a thin aluminium support using
perfluorinated silicon oil. The temperature was maintained at 90 K using an Oxford
TM
Cryosystems Cobra device. Twenty static images were collected at 17° intervals (rotation
perpendicular to the incident beam) with exposure times of one hour. Diffraction images were
indexed to the unit cell obtained from Xꢀray data and data reduction was performed using the
78
Laue1234 software suite. Structure refinement commenced from the nonꢀhydrogen atom
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1
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1
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2
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5
5
6
positions determined from Xꢀray diffraction data. All hydrogen atom positions were located
as large negative peaks in the Fourier difference map and were freely refined using
anisotropic models. Final refinement cycles of 298 parameters against 1211 reflections
(
I>2σI) with no restraints converged at R
analysis was conducted using the program CRYSTALS.
Quantum Mechanical Calculations. All calculations were performed using
1 2
= 0.0398 and wR (all reflections) = 0.0528. TLS
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
7
0
79
80,81
Gaussian 09 programs. Initially calculations were performed using the B3LYP method
with the 6ꢀ311G++(2d,2p) basis set. In order to confirm that the level of theory was sufficient
82
selected calculations were repeated using the M062x method and augꢀccꢀpVTZ basis set.
Differences between the calculated energies determined by the two methods of the
experimental structure and those with different C1―H1 lengths agreed to within 0.1 kcal
ꢀ
1
mol .
RESULTS AND DISCUSSION
1
ꢀ(2,3,5,6ꢀTetrafluoropyridyl)ꢀ3ꢀbenzylimidazolium bromide 1 crystallized from
methanol in the polar space group Pna2 of the orthorhombic crystal system with one cationꢀ
1
anion pair in the asymmetric unit. The structure was determined by a single crystal Xꢀray
diffraction study augmented by a Laue neutron diffraction study to determine the positions of
the hydrogen atoms. Selected distances and angles are given in Table 2. Those involving
hydrogen atoms are given in Table 3. As expected the cation adopts a check mark shape
(Figure 3) with an angle at the methylene carbon atom of 107.68(15)° with the aryl and
fluoroaryl rings nonꢀequidistant. The cation adopts a conformation in which the imidazolium
and phenyl rings are almost perpendicular to the plane defined by N2, C10 and C11 (ca. 91°
and 82° respectively). The cation is chiral, and both enantiomers are present in the crystal
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structure. The tetrafluoropyridyl ring is twisted by ca. 40° from coꢀplanarity with the
imidazolium ring, and is perpendicular to the plane of the phenyl ring. The phenyl ring of one
cation stacks with the tetrafluoropyridyl ring of a cation of opposite configuration. The planes
of these complementary rings are almost parallel, deviating by only ca. 4.8°, with a
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
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29
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32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
†
separation ranging from 3.26(3) (N3···Ar′*) to 3.38(5) Å (ArF*···C15′), and an offset of ca.
6
ꢀ29
1
.4 Å, which is consistent with π–π stacking between arenes and fluoroarenes.
As
predicted, the checkꢀmark shape prevents columnar π–π stacking of the cations. Unlike the
33
structure of 1ꢀ(2,3,4,5,6ꢀpentafluorobenzyl)ꢀ3ꢀbenzylꢀimidazolium bromide, however, the
complementary rings do not form infinite stacks. Instead a bromide anion is positioned close
to the face of the tetrafluoropyridyl ring opposite to the phenyl ring such that
ꢀ
C14′···ArF···Br is almost linear (171.8(3)°) (Figure 4). The only short distance to the face of
the phenyl ring opposite to the tetrafluoropyridyl ring is with one carbon atom of an
imidazolium ring (C13···C3′ 3.358(3) Å), which is nonꢀparallel. The complementary rings
ꢀ
and anion are therefore arranged in C H ···C F N···Br triad stacks. The length of the normal
6
5
5 4
to the plane of the tetrafluoropyridyl ring to the bromide anion is 3.32(3) Å and the bromide
is offset by ca. 0.8 Å towards C5. Similar positions of a bromide anion relative to a
polyfluorophenyl rings have been observed in the crystal structures of
5
1ꢀ56
57,60
pentafluorobenzylammonium and pyridinium bromide salts,
and other salts.
Theoretical calculations using the RIꢀMP2 method and 6ꢀ31++G** basis set, with BSSE
ꢀ
correction, have shown that the optimized threeꢀcomponent C
6
H
6
ꢁꢁꢁC
6
F
6
ꢁꢁꢁBr interaction, for
ꢀ1
ꢀ1
the system with C6v symmetry, is attractive by ꢀ16.1 kcal mol , but that this is 0.4 kcal mol
ꢀ
83
6 6 6 6 6 6
less stable than the sum of separate C H ꢁꢁꢁC F and C F ꢁꢁꢁBr interactions.
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7
8
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1
1
1
1
1
1
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1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
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0
1
2
3
4
5
6
7
8
9
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1
2
3
4
5
6
7
8
9
0
Figure 3. A representative ion pair of 1ꢀ(2,3,5,6ꢀtetrafluoropyridyl)ꢀ3ꢀbenzylimidazolium
bromide (1) from the neutron Laue diffraction structure determination.
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ꢀ
6 5 5 4
Figure 4. The C H ꢁꢁꢁC F NꢁꢁꢁBr motif of 1ꢀ(2,3,5,6ꢀtetrafluoropyridyl)ꢀ3ꢀbenzylimidazolium
bromide (1) viewed perpendicular to the plane of the tetrafluoropyridyl ring. The hydrogen
atoms are omitted for clarity.
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48
49
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60
ꢀ
The C H ···C F N···Br interactions hold the cations in polar chains (Figure 5a). The
6
5
5 4
bromide anion has a close contact with the H3 hydrogen atom of the imidazolium ring in an
adjacent chain. The H3···Br and C3···Br distances, the latter of which is significantly less
8
4,85
than the sum of the van der Waals radius of carbon (1.70 Å)
and the corrected van der
61
Waals radius of the bromide anion (2.35 Å), and the C3―H3···Br angle are indicative of
4
4,45
hydrogen bonding.
The chains are linked by these interactions into sheets parallel to the a
and b axes with polarity parallel to the a axis. The bromide anion also possesses a close
contact with the H1 hydrogen atom of the imidazolium ring of an adjacent sheet. The H1···Br
and C1···Br distances and the C1―H1···Br angle are also characteristic of hydrogen
44,45
bonding.
imidazolium rings and bromide anions, which are in an approximately trigonal planar
environment (C1···Br···C N′ 116.0(2)°, C3···Br···C N′ 122.7(2)°, C1···Br···C3′
21.1(2)°) (Figure 6). Although the sheets have polarity parallel to the a axis, adjacent sheets
The sheets are thus linked by interactions between hydrogen atoms of
6
F
4
5 4
F
1
are arranged antiparallel resulting in no net polarity parallel to the a and b axes. There is,
however, polarity parallel to the c axis arising from the four orientations of the imidazolium
rings. The C N (centroid)―C1 axes of the four different orientations subtend angles with the
3
2
a and b axes of 85.5 and 73.7°, 85.5 and 106.3°, 95.5 and 73.7°, and 95.5 and 106.3°
respectively, and hence any polarity cancels in these directions, but all subtend an angle of
1
63.1° with the c axis (Figure 5b).
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a)
(b)
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Figure 5. The packing of 1ꢀ(2,3,5,6ꢀtetrafluoropyridyl)ꢀ3ꢀbenzylimidazolium bromide (1)
viewed (a) parallel to the c axis showing chains parallel to the a axis held together by
ꢀ
ArꢁꢁꢁArFꢁꢁꢁBr interactions (the hydrogen atoms of the imidazolium ring are omitted for
clarity) and (b) parallel to the b axis showing interactions between the bromide anion and the
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imidazolium rings (the hydrogen atoms of the benzyl group are omitted for clarity).
Figure 6. The environment about bromide in the crystal structure of 1ꢀ(2,3,5,6ꢀ
tetrafluoropyridyl)ꢀ3ꢀbenzylimidazolium bromide (1) showing short interionic distances. The
hydrogen atoms are omitted for clarity.
The C―H bond lengths for all the hydrogen atoms are surprisingly invariant within
experimental error (Table 3). It might be expected that the hydrogen atoms of the
86
imidazolium ring, especially H1, which is the most acidic and readily removed by base,
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would be drawn towards the bromide anions, lengthening the respective C―H bonds, as
44,45
usually occurs with hydrogen bonding.
In order to investigate this, the change in energy
80,81
of an ion pair with C1―H1 distance was calculated using the B3LYP method
and 6ꢀ
311G++(2d,2p) basis set. The calculations (Figure 7) reveal that there is a minimum at 1.105
0
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0
ꢀ1
Å, with a H···Br distance of 2.383 Å, which is only 0.1 kcal mol lower than that of the
actual structure. The C1―H1 and H1···Br distances and C1―H1···Br angle of this
calculated minimum are within 3σ of those experimentally determined. The results also
reveal that variation of C1―H1 between 1.05 and 1.16 Å produces only a small change in
ꢀ1
energy (< 1 kcal mol ). The calculation was also performed to optimize the position of H1,
which resulted in a structure with C1―H1 and H1···Br distances of 1.115 Å and 2.360 Å, a
ꢀ1
C1―H1···Br angle of 169.16° and an energy 0.3 kcal mol lower than that of the
experimentally determined structure. Evidently the interaction has only a small effect on the
C―H bond distance in the crystals studied.
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Figure 7. Variation of energy, relative to the experimental structure, with C1―H1 distance
calculated by the B3LYP method using the 6ꢀ311G++(2d,2p) basis set. The shaded area
represents 3σ of the experimental value.
The energies of interaction between the cation and bromide were calculated by the
80,81
ꢀ1
B3LYP method
and 6ꢀ311G++(2d,2p) basis set to be ꢀ87.5, ꢀ81.2 and ꢀ76.2 kcal mol for
the experimentally determined structures of the ion pairs with the bromide anion close to H1,
ꢀ1
H3 and the tetrafluoropyridyl ring respectively. The first two values are ca. 7 kcal mol more
attractive than those calculated using the MP2 method and the 6ꢀ311G** basis set for similar
4
6
positions of the bromide in the 1ꢀethylꢀ3ꢀmethylimidazolium salt (vide supra), but the
difference between the two energies for the two different systems is similar: 6.3 cf. 6.7 kcal
ꢀ1
mol . For 1ꢀethylꢀ3ꢀmethylimidazolium bromide the energies of interaction were found to be
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almost inversely proportional to the distance from the midpoint of the two nitrogen atoms,
46
which is close to the center of positive charge. Such a relationship is not evident from the
calculations performed on 1: the bromide anion close to H3 is further from the midpoint of
the two nitrogen atoms of the imidazolium ring than that close to the tetrafluoropyridyl ring
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ꢀ1
(4.93 cf. 4.71 Å) and yet has an interaction that is calculated to be 5 kcal mol stronger. Since
ꢀ
50,61ꢀ64
the C
5
F
4
N···Br is not expected to be repulsive,
we conclude that the interactions
cannot be considered exclusively electrostatic interactions between the anion and the center
of charge of the cation. A calculation was also performed to optimize the position of the
bromide anion close to H1. This resulted in a structure in which the bromide anion is still in
the plane of the imidazolium ring, but shifted towards C10 to give an H1···Br distance of
2
.230 Å and a C1―H1···Br angle of 154.69°. This structure was calculated to be 2.2 kcal
ꢀ1
mol lower in energy than that of the structure determined experimentally. As a result it is
ꢀ
ꢀ
tentatively suggested that, although the H1···Br and H3···Br interactions may not be
conventional hydrogen bonds, the orientation dependence of the interaction may be greater
4
6
than previous calculations suggest.
ꢀ
In summary in the crystal structure of 1 the C
6
5 5 4
H ···C F N···Br stacking involving the
complementary rings of adjacent cations generates polar chains which arrange in sheets
which are connected by interactions between acidic hydrogen atoms of imidazolium rings and
bromide anions. The sheets are antiparallel, but the orientation of the imidazolium rings gives
rise to polarity parallel to the c axis and hence the polar space group.
In order to examine further the importance of the interactions between the imidazolium
hydrogen atoms and the bromide anions the crystal structures of the salts in which the
hydrogen atoms H1 and H3 had been replaced by methyl groups were investigated. 1ꢀ
(2,3,5,6ꢀTetrafluoropyridyl)ꢀ2ꢀmethylꢀ3ꢀbenzylimidazolium bromide (2) crystallized in the
ꢀ
1
space group P
of the triclinic crystal system and 1ꢀ(2,3,5,6ꢀtetrafluoropyridyl)ꢀ3ꢀbenzylꢀ4ꢀ
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methylimidazolium bromide (3) crystallized of the space group P2 /c of the monoclinic
1
crystal system. The cations of 2 (Figure 8) and 3 (Figure 9) possess very similar geometric
parameters to 1 (Table 2), with the exceptions that the tetrafluoropyridyl rings of 2 and 3 are
twisted ca. 20° more from coꢀplanarity with the imidazolium ring than for 1, and the phenyl
ring of 2 is positioned such that the plane of the imidazolium ring and that defined by N2,
C10 and C11 are close to coꢀplanar; they are close to parallel for 1 and 3. Whilst the planes of
the phenyl and tetrafluoropyridyl rings of 1 are approximately orthogonal, those of 2 are
close to parallel and those of 3 lie at ca. 60° to each other.
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Figure 8. Two stacked ion pairs of 1ꢀ(2,3,5,6ꢀtetrafluoropyridyl)ꢀ3ꢀbenzylꢀ2ꢀ
methylimidazolium bromide (2). The hydrogen atoms are omitted for clarity.
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Figure 9. Structure of 1ꢀ(2,3,5,6ꢀtetrafluoropyridyl)ꢀ3ꢀbenzylꢀ5ꢀmethylimidazolium bromide
(3). The hydrogen atoms are omitted for clarity.
The crystal structure of 2 comprises pairs of cations, related by a centre of inversion,
ꢀ
held together by two C
6
H
5
···C
5
F
4
N···Br interactions. The bromide and the midpoint of
C13ʹ―C14ʹ lie on opposite faces of the tetrafluoropyridyl ring on the normal from C8
ꢀ
(
C13ʹ―C14ʹmidpoint···C8···Br 177.1(3)°) (Figure 10). The planes of the two interacting rings
subtend an angle of 17.3(3)°. In addition to that interacting with the tetrafluoropyridyl ring,
two other bromide anions are close to the cation. One is positioned close to C2, in the plane
of the imidazolium ring, with the geometric parameters (Table 2) suggesting hydrogen
bonding. The other is positioned on one face of the imidazolium ring, 3.36 Å along the
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normal to the midpoint of C1―N2. Although the planes of adjacent imidazolium rings are
parallel and separated by just 3.44(3) Å, the rings are displaced by ca. 1.9 Å which prevents a
π–π stacking interaction, or avoids repulsion, between them.
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ꢀ
Figure 10. The C
6
H
5
ꢁꢁꢁC
5
F
4
NꢁꢁꢁBr motif of 1ꢀ(2,3,5,6ꢀtetrafluoropyridyl)ꢀ3ꢀbenzylꢀ2ꢀ
methylimidazolium bromide (2) viewed perpendicular to the plane of the tetrafluoropyridyl
ring. The hydrogen atoms are omitted for clarity.
ꢀ
The cations of 3 are arranged in polar chains connected by C
6
H
5
···C
5
F
4
N···Br stacks
involving cations of opposite configuration similar to the crystal structure of 1. The planes of
the complementary rings deviate from parallel by ca. 9.4° with a separation ranging from
3
.386(4) Å (N3···Ar′) to 3.669(4) Å (ArF···C14′). The length of the normal from the plane of
the tetrafluoropyridyl ring to the bromide anion is 3.32(3) Å and the bromide is offset by 0.96
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ꢀ
Å towards the midpoint of C4―C9 (Figure 11). The Ar···ArF···Br angle is almost linear
173.2(3)°). The bromide anion also possesses a close contact with the H1 hydrogen atom of
the imidazolium ring of an adjacent chain. The C1···Br distance and the N―C1···Br angles
(
4
4,45
(Table 2) are suggestive of hydrogen bonding.
The interaction forms sheets parallel to the
0
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a and c axes with polarity parallel to the c axis (Figure 12). The other imidazolium hydrogen
atom, H2, is positioned close to the bromide anion adjacent to the tetrafluoropyridyl ring
(
C2···Br 3.683(3) Å). The lack of a hydrogen atom in the 4 position prevents further
hydrogen···bromide interactions, and consequently there is no strong interaction between the
sheets, which align antiꢀparallel and so with no net polarity.
ꢀ
Figure 11. The C
6
H
5
ꢁꢁꢁC
5
F
4
NꢁꢁꢁBr motif of 1ꢀ(2,3,5,6ꢀtetrafluoropyridyl)ꢀ3ꢀbenzylꢀ4ꢀ
methylimidazolium bromide (3) viewed perpendicular to the plane of the tetrafluoropyridyl
ring. The hydrogen atoms are omitted for clarity.
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Figure 12. The packing of 1ꢀ(2,3,5,6ꢀtetrafluoropyridyl)ꢀ3ꢀbenzylꢀ4ꢀmethylimidazolium
bromide (3) viewed parallel to the a axis showing chains parallel to the c axis held together
ꢀ
by ArꢁꢁꢁArFꢁꢁꢁBr interactions. The hydrogen atoms are omitted for clarity.
ꢀ
Since the structures of 1, 2 and 3 all contain C
6
H
5
···C
5
F
4
N···Br motifs we were
prompted to investigate whether this interaction occurs for other polyfluoroarylꢀ and arylꢀ
substituted imidazolium salts. The structures of
phenylbenzyl)imidazolium bromide (4) and
1ꢀ(2,3,5,6ꢀtetrafluoropyridyl)ꢀ3ꢀ(3ꢀ
1ꢀ(2,3,5,6ꢀtetrafluoropyridyl)ꢀ3ꢀ(2ꢀ
naphthylmethyl)imidazolium bromide (5), which bear the same polyfluoroaryl substituent as
, but a more elaborate benzyl substituent, and 1ꢀ(2,3,4,5,6ꢀpentafluorobenzyl)ꢀ3ꢀ
1
6
5
phenylimidazolium bromide (6), in which the positions of the aryl and polyfluoroaryl
substituents are switched, were then determined. Salt 4 crystallized as the monohydrate from
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6
methanol in the centrosymmetric space group P2 /c with one cationꢀanion pair and one water
1
molecule in the asymmetric unit. Salt 5 crystallized also as the monohydrate from methanol
1
in the polar space group P2 with four cationꢀanion pairs and four water molecules in the
asymmetric unit. The crystal from which the structure was determined was pseudoꢀ
merohedrally twinned with additional racemic twinning. Salt 6 crystallized from
dichloromethane in the centrosymmetric space group Pbca in the orthorhombic crystal
system with one cationꢀanion pair in the asymmetric unit.
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0
The cations of 4, 5 and 6 exhibit a similar check mark shaped geometry to 1 (Figures 13
–
15), with similar bond distances and angles for the imidazolium ring and methylene group
(Table 2). As in the structure of 1 there is a close contact between a bromide anion and H1 of
the cations, with C1···Br distances significantly less than the sum of the van der Waals radius
61,84,85
of carbon and the corrected van der Waals radius of bromide (4.05 Å
) and an
approximately trigonal planar arrangement of N1, N2 and the anion about C1 (Table 2),
indicating a hydrogen···bromide interaction.
Figure 13. Structure of 1ꢀ(2,3,5,6ꢀtetrafluoropyridyl)ꢀ3ꢀ(3ꢀphenylbenzyl)imidazolium
bromide monohydrate (4). The hydrogen atoms are omitted for clarity.
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Figure 14. Structure of one of the ion pairs of 1ꢀ(2,3,5,6ꢀtetrafluoropyridyl)ꢀ3ꢀ(2ꢀ
naphthylmethyl)imidazolium bromide monohydrate (5). The hydrogen atoms are omitted for
clarity.
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